Here's how to set that.
Everybody, listen up. If you don't know about this, you should probably pay attention to the following paragraph, which you may find important one of these days...
The trick is to remember that when you create a new, blank document, Word pulls the settings for that new document from the Normal template. So if you want to make changes "stick" for new documents, you need to make the changes for that setting in the Normal template (assuming you're not using some other template).
The other trick is that you need to open the Normal template itself, rather than creating a new blank (untitled) document.
Here's how:
1. Launch Word
2. In Word, click File > Open, then navigate down to the Normal template, which is named "Normal.dotm" and is located here:
Users / [UserName] / Library / Application Support / Microsoft / Office / User Templates / Normal.dotm
Note; Be sure you open this template by using Word's menus; do not simply double-click on the template in the Mac OS Finder (that won't open the template itself, it will open a new blank document based on the template, and your changes won't stick).
A "blank" document should open. Be sure that the document title says "Normal.dotm", not "Untitled".
3. In the Normal.dotm template, click View > Toolbars > Drawing.
4. On the Drawing Toolbar, click the Lines button, then click the Line tool (first icon on the fly-out).
5. In the document, click and drag to draw a line. It doesn't matter what the line looks like.
6. With the line still selected, click Format > AutoShape
7. In the Format AutoShape dialog, make any changes you want. In the case of your specific question (you want to make the default line Black and .75 pt), on the Colors and Lines tab, under Line, change the Color to Black and the Weight to .75. Click OK.
8. Back in your document, right-click (or Ctrl+click) the line you just drew and on the contextual menu that pops up, click Set AutoShape Defaults.
9. Delete the line you just drew. Yes, delete it...it has served its purpose (which was to allow you to tweak the settings), and if you don't delete the line, every new document you create will have that line in it.
10. Click File > Save to save the Normal.dotm template.
11. Close the Normal.dotm template.
Now, every time you create a new, blank document, the default line should have the properties you just set.
Hope that helps.